The present invention relates to networks of switches comprising N input accesses and M output accesses, adapted to connect P (P≦N and P≦M) input accesses, selected from the N input accesses, to P output accesses selected from the M output accesses.
Such networks of switches are for example used in satellite payloads, in particular of transparent telecommunications satellites, where they are generally placed upstream of or downstream from equipment such as transponders.
In general, two networks of switches are used on either side of the equipments: one network comprising N input accesses and M output accesses, upstream of the equipments and an inverted network downstream from said equipments, i.e. the same network, with the input accesses and output accesses inverted (i.e. M input ports and N output ports). Indeed, in such networks of switches, the input and output accesses are interchangeable in practice.
Such networks of switches, when they are implemented in telecommunications satellites, are adapted to establish P communication channels from among N installed channels by means of P transponders from among M available transponders.
Such networks of switches make it possible for example to deal with failures of at most (M-P) transponders (in cases where M is greater than N) by altering the paths of the P input signals in the network such as to route them exclusively towards equipment in working order.
Currently, the switches used in telecommunications satellites' on-board networks comprise four ports, and are most often three-state or four-state switches (these states are generally known as “positions”).
FIGS. 1a, 1b and 1c represent respectively the three states of a three-state switch cT:                one state wherein a first port x1 is connected to a second port x2 and a third port x3 is connected to a fourth port x4;        one state wherein the first port x1 is connected to the third port x3 and the second port x2 is connected to the fourth port x4;        one state wherein the first port x1 is connected to the fourth port x4 and the second port x2 is connected to the third port x3.        
FIGS. 2a, 2b 2c and 2d represent respectively the four states of a four-state switch cQ):                one state wherein a first port x1 is connected to a second port x2 and a third port x3 is connected to a fourth port x4;        one state wherein the first port x1 is connected to the third port x3;        one state wherein the first port x1 is connected to the fourth port x4 and the second port x2 is connected to the third port x3;        one state wherein the second port x2 is connected to the fourth port x4.        
It is understood therefore that, by altering the state of each switch, it is possible to modify the ports effectively connected with each other for each switch and, consequently, to modify the choice of the P input accesses and P output accesses that are mutually and individually connected.
Within the field of telecommunications satellites, the design of a network of switches requires a complex study aimed at determining both a logical configuration (i.e. the number of switches and which ones of these are connected to each other) and a physical installation (i.e. the final arrangement of the switches and of the linking elements between the switches), which meet the operating constraints, in particular of small size.
The design of networks of switches is increasingly complex, particularly because of the increase in:                the number of transponders installed in a single satellite, some of said transponders being redundant transponders provided to deal with possible failures,        the flexibility required for these networks, i.e. their ability to simultaneously establish many different combinations of paths between their input accesses and their output accesses.        
It is also understood that the flexibility and size constraints are conflicting constraints and the result of the design study is usually a compromise between these two constraints.
A matrix network of switches is known from patent application EP 0876069, derived from networks of switches used in terrestrial telecommunications. This network of switches can be built modularly.
However, a drawback of the network of switches described in patent application EP 0876069 is that the number of switches increases rapidly as the number of input accesses and output accesses is increased. In addition to this network's size, the average number of switches passed through to establish the P paths in the network will also increase. In practice, it is important to reduce the average number of switches passed through, as a signal is attenuated by each passage of this signal through a switch.